In the public chat, you will always see a message like the following at the top: "To join this meeting by phone, dial: +4930 123 456 789 and enter the following PIN: 12341234123#. Use the 0-key on your phone to toggle mute on/off". You need the phone number and the PIN. If you can't get into the meeting and chat yourself, have someone in the meeting send it to you. Once you're in, you can mute or unmute yourself using the 0 key.